The present invention relates to heterojunction bipolar transistors widely used for high power amplifiers for transmission, and methods for fabricating such heterojunction bipolar transistors.
With recent functional enhancement of cellular phones and increase of capacity in communication with cellular phones, high-frequency analog devices for use in cellular phones have been required to have further enhanced performance. Out of these high-frequency analog devices, heterojunction bipolar transistors (hereinafter, referred to as HBTs) have been already put into practical use as high power amplifiers for transmission. To enhance the performance of the HBTs, the emitter size needs to be reduced.
Hereinafter, a structure of a conventional HBT and a method for fabricating the HBT will be described with reference to FIG. 23 (see, for example, Daisuke Ueda et al., “high-frequency optical semiconductor device”, The Institute of Electronics, Information and Communication Engineers, Dec. 1, 1999, pp. 51 to 53).
FIG. 23 is a view schematically illustrating a cross-sectional structure of a conventional HBT. As illustrated in FIG. 23, a sub-collector layer 302 made of a high-concentration n-type GaAs layer is formed on a semi-insulating substrate 301 made of GaAs. A collector layer 303 made of a low-concentration n-type GaAs layer and a base layer 304 made of a high-concentration p-type GaAs layer are formed on a collector region of the sub-collector layer 302. An emitter layer 305 made of an n-type AlGaAs layer, an emitter contact layer 306 made of a high-concentration n-type InGaAs layer, a first emitter electrode 314 made of a WSi layer and a second emitter electrode 311 having a Ti/Pt/Au structure (i.e., a structure in which a Ti layer, a Pt layer and an Au layer are stacked in this order: the same hereinafter) are formed as a stack on an emitter region of the base layer 304. A base electrode 312 having a Ti/Pt/Au structure is formed on a region of the base layer 304 except for the emitter region. A collector electrode 313 having an AuGe/Ni/Au structure (i.e., a structure in which an AuGe layer, a Ni layer and an Au layer are stacked in this order) is formed on a region of the sub-collector layer 302 except for the collector region.
As illustrated in FIG. 23, to electrically isolate HBTs (unit HBTs) from each other, an isolation region 321 is formed in the periphery of each HBT region to reach the semi-insulating substrate 301 through the sub-collector layer 302.
As illustrated in FIG. 23, an insulating film 322 is deposited over the entire surface of the semi-insulating substrate 301 having an HBT structure. Contact holes 323A, 323B and 323C are provided in parts of the insulating film 322 located on the second emitter electrode 311, the base electrode 312 and the collector electrode 313, respectively. An interconnection 315A is extended from the second emitter electrode 311 through the contact hole 323A. An interconnection 315B is extended from the base electrode 312 through the contact hole 323B. An interconnection 315C is extended from the collector electrode 313 through the contact hole 323C.